I don't agree. I buy a sight stone, but then also supplement buying wards because sometimes 2 wards just aren't enough at one time. I like having 4 or 5 wards out. 2 in my lane, 1 in bush by river, 1 in dragon, and sometimes 1 in the enemy junglers ramp to river. Vision is OP.

As support a Ruby Sightstone is rarely enough in a game. You should be buying wards in addition to Sightstone, especially since Stone doesn't provide Pinks. But honestly, you're probably wasting a lot of money.

It is rare that you should ever need both lane bushes warded. You need the relevant or contested bush warded, the only reason you'd ever need both bushes warded is if the contested bush changes and you already have a ward in the other bush.

If you are placing a ward in river bush and under dragon you are probably throwing away a lot of possible vision advantage. Try warding near dragon in a place that gives you vision of the enemy jungle exits, then use the other ward in the enemy's jungle. After warding dragon or near dragon during the laning phase I will often stick a second ward up in the enemy's jungle. This gives you amazing advantages. First of all if you ward blue buff you give your team a lot more buff control and help your jungler counterjungle better. The biggest advantage though is probably the fact that your top lane can function a lot smoother, especially when pushing, if they know what half of the jungle the jungler is in. Top can even help counterjungle if they have good vision. Warding areas like the jungle where you know the enemy will be means that your wards will do a lot of work, they will provide a constant advantage to every lane.

Also if you take Clairvoyance and use it at the right times and in the right locations you can get a pretty good advantage that way. However, a summoner spell slot is a pretty steep price, especially since something like Exhaust can give you another escape on top of Flash and be used to give kills to or save your laning partner.

On the primary topic, you are correct in saying that other positions should buy wards. They shouldn't be spamming wards, but they should always at least have a ward in their inventory in case they are put in a risky position by pushing, laning at low health, or having low amounts of data on the enemy team's position.

I've got ward support in both Runes and masteries so I can keep vision up. Sight stones are useful, but I find it's too easy to forget to pick up normal wards with them. I've semi-solved that by grabbing pink wards for a combination of quick wards (Sightstone/Ruby Sightstone) and vision to spot enemy wards at dragon, baron, and buffs.

This just comes back around to the personal responsibility lacking in this community (and in the greater gaming community in general). It seems that most Summoners constantly want to be "the best player", not the best team. This thought process usually drives them to focus on their own personal "best build" at the cost of the team, usually in the form of either not buying wards (because it costs them money) or buying redundant or useless items (an Abyssal on the AP Carry when your jungle Amumu already has one or Black Cleavers when the enemy aren't building any armor). They then blame anything they lack on the rest of the team (lack of vision is the Support's fault, despite the Support covering their half of the map well; lack of ganks on the Jungler, despite keeping the enemy pushed to their turret; etc). People just don't want to admit to their mistakes.

With that said, warding IS everybody's responsibility, but, due to the nature of the roles, the burden of it does fall on the Support and Jungler. The Support, assuming (s)he has enough GP/10 from Items, Masteries, and Runes, can usually afford runes the easiest because their builds are usually cheaper. The Jungler, though often stretched on gold in the current iteration of the jungle, has the largest map presence, and therefore should be getting wards into places inconvenient for the Support, such as the top half of the map or in the enemy jungle.

Top is no-mans-land if you think you can get away with not buying wards then don't. If you want to be absolutely sure you aren't getting ganked then buy them. It is always the top laner's digression whether to buy wards or not.

early game, support and jungler should be buying wardsMid and ADC need to be building in order to win trades that are bound to happen

mid game, support jungler and mid should be buying wardsMid should start buying wards to prevent ganks that are bound to happen

late game, everyone should be buying wardsEveryone should be pitching in to ward river and jungle entrances so that your team doesn't get caught out

Support should always buy the red ward stone, depending on what kind of champ they are it might be good for top or jungle to buy it as well, if health is a major part of their build then they should build it.

I echo the remarks by Boagster. I would add that junglers, tanks and supports can't be broad-brushed any longer.

There is much less gold received by an assist than before, so players that tank and rack up assists are well behind. 1300 gold for a jungler to get the ruby sightstone is a large investment early and mid game.

Returning regularly to purchase wards also makes less sense for the slower players. They lose valuable ganking and farming time due to their glacial travel speeds. Seriously, once you have the IE and one PD, it's not going to kill you to get a few wards. It might even save your life.